As I mentioned to someone else, I don't count incrementals. 6.5 is mainly an incremental update that caters more towards their cloud subscribers. If I counted incrementals, then CS5.5 would have been included as well (which mainly introduced the subscription method to Adobe users). Existing CS5 users received the same updates that 5.5 offered in a form of a patch.

Think of 6.5 as a mild refresh between full makeovers. If they go that route on a yearly basis, then full version changes may be pushing closer to the 24 month mark.